IGCSE Biology Paper-4: Specimen Questions with Answers 141 - 142 of 279

Passage

Cotton, Gossypium hirsutum, is grown for the fibers that form within the fruits after fertilization, as shown in Fig.

Fibers from the fruits of cotton plants are used in the textile industry.

The Fibres
  • Cotton plants have been genetically engineered to produce a protein that is toxic to the caterpillars of several insect pests. This gives the cotton plants resistance to the pests.
  • The cry gene for pest resistance was isolated from the bacterium Bacillus thuringiensis and inserted into the cells of cotton plants as shown in Fig.

Bacterial DNA

Question 141 (4 of 5 Based on Passage)

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An enzyme cuts the cry gene from the DNA of B. thuringiensis.

Explain how the DNA is inserted into the plasmid.

Explanation

In a typical DNA cloning procedure, the gene or other DNA fragment of interest is first inserted into a circular piece of DNA called a plasmid. The insertion is done using enzymes that “cut and paste” DNA i. e. DNA ligase, and it produces a molecule of recombinant DNA, or DNA assembled out of fragments from multiple sources.

Molecule of Recombinant

Question 142 (5 of 5 Based on Passage)

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An enzyme cuts the cry gene from the DNA of B. thuringiensis.

(i) State the name of the enzyme that cuts DNA.

Explanation

Restriction Endonucleases

  • They cut the DNA strands at specific base sequence in palindrome site. Thus, they function as ‘molecular scissors’ or chemical scalpels. The restriction endonuclease inspects the length of a DNA sequence. Once it recognizes specific sequence, it binds to the DNA and cuts each of the two strands of the double helix at specific points in their sugar phosphate back bones. Special sequence in the DNA recognized by restriction endonuclease is called palindromic nucleotide sequence.
  • The palindromes are groups of letters that form the same words when read in both directions forward and backward. Ex: